1. Field of the Invention
The present invention relates to a voltage regulator that outputs a constant output voltage, and more particularly, to a phase compensation circuit that stabilizes an operation of a voltage regulator.
2. Description of the Related Art
FIG. 3 is a circuit diagram illustrating a conventional voltage regulator.
The conventional voltage regulator includes an output transistor 21 that outputs an output voltage Vout, a voltage divider circuit 22 that divides the output voltage Vout, and a reference voltage circuit 23 that generates a reference voltage. The conventional voltage regulator also includes an error amplifier circuit 24 that controls the output transistor 21 so as to hold the output voltage Vout constant on the basis of the output voltage of the voltage divider circuit 22 and the reference voltage, and a phase compensation circuit 20 that is disposed between the output transistor 21 and the error amplifier circuit 24, and compensates a phase of an output terminal 20d of the phase compensation circuit 20. The phase compensation circuit 20 has a phase compensation capacitor 20a and a phase compensation resistor 20b (for example, see JP 2005-215897 A).
In the phase compensation circuit 20 of the voltage regulator, a resistance of the phase compensation resistor 20b may be set to be large in order to achieve a stable operation of the voltage regulator.
When the output voltage Vout of the voltage regulator changes, an output voltage of the error amplifier circuit 24 also changes. In a transient state in which the output voltage of the error amplifier circuit 24 changes, when the resistance of the phase compensation resistor 20b is large, it takes time to charge and discharge a gate of the output transistor 21.
FIGS. 4A and 4B are graphs each illustrating an input voltage and an output voltage, respectively, of the phase compensation circuit of the conventional voltage regulator.
When an input voltage V1 of the phase compensation circuit 20 changes as illustrated in FIG. 4A, an output voltage V2 of the phase compensation circuit 20 changes as illustrated in FIG. 4B. When the resistance of the phase compensation resistor 20b is small, the output voltage V2 changes as indicated by a dotted line in FIG. 4B. On the other hand, when the resistance of the phase compensation resistor 20b is large, the output voltage V2 changes as indicated by a solid line in FIG. 4B.
That is, there arises such a problem that transient response characteristics of the phase compensation circuit 20 are deteriorated, and therefore the transient response characteristics of the voltage regulator are deteriorated.